The present invention relates to optical devices and systems, and in a particular embodiment, provides a Mach-Zehnder interferometer based device which may be used as an optical circulator or isolator.
An optical isolator is a nonreciprocal device which allows the passage of light in only one direction. A signal transmitted in a forward direction through a first port of an optical isolator will be passed to a second optical port. However, optical signals traveling in a rearward direction through the second optical port are blocked by the optical isolator from reaching the first port. Such optical isolators have found a wide variety of uses in optical systems, particularly those using optical fibers.
An optical circulator is a nonreciprocal optical device related to the optical isolator. Optical circulators allow the passage of light from a first port to a second port, as in an optical isolator. However, rather than simply blocking signals traveling in a reverse direction into the second port, such signals are instead transmitted to a third port. Any two consecutive ports of an optical circulator are, in effect, an optical isolator since signals travel in only one direction between the ports.
Circulators will generally have three or more ports. Light transmitted into the first or second port of a three port circulator will be directed to the next higher number port. In a closed circulator, light transmitted into the third (or other highest number port) is passed to the first port. In an open three port circulator, light directed into the third port will be blocked by the circulator without transmitting the light to any other active port. Regardless, the function performed by the circulator is called a circulating operation.
Several types of optical circulators have been developed. The structure of a conventional optical circulator includes three basic components: polarization beam splitters (PBSs), nonreciprocal Faraday rotators, and half-wave plates. Each beam splitter typically includes at least one optical deflection element such as a prism. Assembly of these conventional circulators is fairly difficult, so that the cost of conventional circulators is quite high.
Much work has gone into improving the performance of optical circulators. While conventional circulators provide an isolation of about 30 dB, additional birefringent crystals may be employed to improve isolation to over 40 db. Exemplary bifrefringent enhanced optical circulators are commercially available from E-Tek Dynamics, Inc. of San Jose, Calif., and related devices may also be available from Nippon Telegraph and Telephone Corporation of Japan, FDK America, Inc., of California, and other sources. Generally, circulators which include both a conventional polarization beam splitter and birefringent crystals will have costs higher than those of a conventional circulator.
Optical circulators based on light path deflection of birefringent polarizers have also been proposed and implemented. These birefringent polarizer based structures have enhanced isolation performance, but often at a substantially higher cost. Moreover, optical circulators based on either polarization beam splitters or birefringent polarizers are susceptible to polarization mode dispersion (PMD) if there is a lack of symmetry between the optical paths of the separated beams. Such polarization mode dispersion can limit the signal transmission speed of an optical network, while the symmetrical circulator structures proposed to date are often very difficult to align and/or include highly specialized optical elements. Once again, exemplary birefringent polarizer based optical circulators are commercially available from E-Tek Dynamics, while competing structures may be available from Nippon Telegraph and Telephone Corporation of Japan, JDS Fitel, Inc., of Canada, Photonic Technologies of Australia, and others.
The incremental improvements in high performance circulators have provided a variety of options for applications requiring high isolation with low insertion loss. Unfortunately, the cost of each circulator structure is often prohibitive for applications requiring numerous circulators. Moreover, there are applications for the optical circulating operation which do not require the performance of these costly structures. For example, in fiber optic networks, relatively low cost amplification is available to overcome a relatively large amount of insertion loss.
A recent paper published by T. Shintaku et al. of NTT Opto-electronics Laboratories of Japan, describes a waveguide polarization-independent optical circulator based on a Mach-Zehnder interferometer. This structure combines two 45.degree. Faraday rotators and two half-wave plates with a Mach-Zehnder interferometer structure. A Faraday rotator and a half-wave plate are aligned symmetrically along each leg of the interferometer, and the resulting circulator is described as providing an isolation of between 14.1 and 23.7 dB with an insertion loss of between 3.0 and 3.3 dB.
While the recently proposed Mach-Zehnder interferometer based optical circulator appears to provide a useful alternative to circulators based on conventional polarization beam splitters, birefringent crystal enhanced polarization beam splitters, and birefringent crystal polarizers, particularly when the cost of these structures is not justified. Nonetheless, it would be desirable to provide still further improvements in optical circulators, and in optical circulation methods. It would be particularly desirable to provide optical circulator structures having improved manufacturability and still lower cost, while maintaining acceptable isolation, insertion loss, polarization mode dispersion, and polarization dependent loss characteristics. It would further be desirable if these improvements were applicable to fiber based optical circulators, integrated optical element systems, table top optical networks, optical isolators, and the like.